A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size (critical dimension) of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength λ and, thus, reduce the critical dimension (CD), an extreme ultraviolet (EUV) radiation may be used. EUV radiation may be considered to be radiation having a wavelength within the range 4-20 nm. A lithographic apparatus which uses EUV radiation, may be used to form smaller features on a substrate than a lithographic apparatus which uses radiation of a longer wavelength (e.g. a wavelength of approximately 193 nm). EUV radiation may be generated using a laser produced plasma radiation source or some other EUV radiation source. A radiation source and a lithographic apparatus may together be referred to as a lithographic system.
It is desirable to provide a radiation source and a lithographic system which obviates or mitigates one or more problems arising from the prior art.